Method for controlling the thickness of polymeric film structures

ABSTRACT

A method is provided for manufacturing film structures of organic thermoplastic polymeric materials of improved thickness uniformity across the width thereof. Briefly, the method comprises measuring the thickness of a film structure at any location across the width thereof before stretching, measuring the thickness of said film structure at any location across the width thereof after stretching, computing the cumulative mass values of said film structure across the width thereof before and after stretching, and adjusting the thickness of said film structure at any point across the width thereof before stretching responsive to deviation from a preselected value in thickness of said film structure after stretching corresponding to the cumulative mass value of said film structure before and after stretching.

March 14, 1972 F. L. KNOWLES 3,649,726

METHOD FOR CONTROLLING THE THICKNESS OF POLYMERIC FILM STRUCTURES FiledSept. 20, 1968 2 Sheets-Sheet 1 FIG-I INVENTOR FREDERICK LYNN KNOWLESATTORNEY March 14, KNOWLES METHOD FOR CONTROLLING THE THICKNESS OFPOLYMERIC FILM STRUCTURES 2 Sheets-Sheet 3 Filed Sept. 20, 1968 3 wz:322222 a m QN.

.3: 53 Q unhs 3 :5 822222 Q N. a s5 M mv 1 l NOllVl/HG i0 O o o o o o oo O o o 2 a; co k 0 m =1- ro N SSVI J0 BAIIV'IHNHO dz $33 322222 a w vSE 3 Eu h on M N --uu 0: w 3 2 2 3 N o w Q N o 2 554mm; 2 V. 0 I W n m 73 M H H 3 a M 8 w E X A I n on W 8 2: W Qfu INVENTOR FREDERICK LYNNKNOWLES BY J. Hana/41M ATTORNEY United States Patent Otfice US. Cl.254-40 8 Claims ABSTRACT OF THE DISCLOSURE A methodis provided formanufacturing film structures of, organic thermoplastic polymericmaterials of improved thickness uniformity across the width thereof.Briefly, the method comprises measuring the thickness of a filmstructure atany location'across the width thereof before stretching,measuring the thickness of said film structure at any location acrossthe width thereof after stretching, computing the cumulative mass valuesof said film structure across .the width thereof before and afterstretching, and adjusting the thickness of said film structure at anypoint across the width thereof before stretching responsive to deviationfrom a preselected value in thickness of said film structure afterstretching corresponding to the cumulative mass value of said filmstructure before and after stretching.

The present invention relates to a method for processing film structuresof organic thermoplastic polymeric materials and, more particularly, isdirected to a method for processing polymeric structures of improvedthickness uniformity or gauge across the width thereof.

,In .the manufacture of continuous lengths of film structures ofpolymeric materials it is very dilficult to produce film structuresthatare uniformly fiat from edge to edge. Ordinarily, thicker or thinnerlanes appear across the width of thefilm structures which render suchfilm structures unsuitable for many uses and, furthermore, results inpoor roll formation when winding the film structures'into roll form. Theabove-mentioned thickness variation not only includes gradual changes inthickness froinedge-to-edge of the film structures, but also includesrelatively narrow bands wherein the thickness of the filmchangesabruptly from the average at that point or from a preselectedvalue. Numerous apparatus and techniques have evolved to sense thicknessdeviations and effect adjustment of film thickness at specific locationsusually at or near the point of extrusion of the polymeric material inthe film forming operation. For instance, US. Pat. Nos. '3,l22,7 82 and3,122,783 describe mechanical arrangements for adjusting the thicknessof selected hypothetical lanes across a film structure corresponding tothicker or thinner portions of the film. Specifically, US. Pat. No.3,122,782 describes the application of twointercon'nectedmultiple-junction terminal boards disposed intermediate afilm thickness sensing gauge and adjustment means" for'changingthelwidth of the extrusion orifice from whence the film structure issues.The film thicknessgauge measures the thickness of the film structure'and transmits abias signal through suitable relay elements forenergizing motors which are so constructed and arranged to cause openingor closing of the orifice opening in theextruder in accordance with thedeviation in the thickness of the film that is sensed by the filmthickness measuring gauge. The bias signal is adjusted to selectivelyenergize. one-of many motors thereby to restrict the adjustment of theextruder orifice over only a localized portion of the entire orificestructure. Adjustment of the bias signal is accomplished by means of thetwo interconnected multiple-junction terminal boards, wherein the firstterminal boardisloperatively connected 3,649,726 Patented Mar. 14, 1972to the film thickness measuring gauge and the second terminal bard isoperatively connected through the aforementioned relay elements to themotors for adjusting the extruder orifice opening. The first terminalboard contains a greater number of junctions, corresponding in number tothickness sensing points and hypothetical longitudinal lanes in theextruded film structure, than the secend terminal board so that filmthickness deviations from several sensing points are transmitted throughthe interconnected terminal boards to a single adjustment point. Anothermethod for improving the thickness uniformity of polymeric filmstructures is described in US. Pat. No. 3,161,711 which disclosesheating selected longitudinal lanes of greater thickness of amorphouspolymeric films followed by cooling the entire film whereby to effect amore marked temperature difierential between the thinner and the thickerlongitudinal lanes of the film structure, followed lastly by stretchingthe unsupported film structure thereby to obtain a film structure ofimproved thickness uniformity. I

Identifying the precise location at the source or origin of the filmstructure as at the extruder orifice that corresponds tothe location ofthe film thickness deviation sensed at a point remote therefrom is verydifficult. The difficulty is especially apparent in film structureswhich are stretched after being cast or extruded, with the result thatunpredictable lateral migration of characteristics or features oftenoccurs as the film structure passes from the point of origin (extrusion)to the point of film thickness sensing at or near the end of the filmmaking process. The net result is that adjustments are frequently madeat unwanted locations, causing a magnification rather than a decrease ingauge deviation. Thus, automatic control apparatus adapted for theadjustment of film thickness having a thickness sensing device whichtransversely scans the moving web near the finished end, transmitting asignal for appropriate apparatus adjustment near the origin of the film,is severely handicapped and often totally frustrated especially whenintermediate stretching oper ations are introduced which preventadjustments at the correct points in order to nullify or minimize thedeficiency sensed at the remote location. The foregoing is particularlytrue as regards the correction of thickness deviations of filmstructures extruded from fiat dies and which are biaxially stretched inmutually perpendicular directions. Often the extrusion die does not haveparallel lips, but lips which are spaced apart to extrude a contouredfilm of lenticular cross-section having heads at each edge adapted to begripped by clips for transverse stretching; the lenticular cross-sectioncompensates for the greater extent to which the center portion of thefilm stretches when the film is stretched in its longitudinal direction.The degree to which each increment of film across the width changes inthe stretching process as a function of the purposely constructedcontour can be estimated with fair accuracy, but the non-uniformities inthickness aris ing from 'flow irregularities. through the extruder cancause irregularities in the stretching pattern so that an,

increment of film width is displaced across the film to an unpredictabledegree. Further, it is difficult, if not impos sible, to correlate thelocation of a deviation on the stretched film to a location on thefilm-prior to stretching.

According to the present invention, there is provided an apparatushaving means operable on a moving film structure adapted to be subjectedto a transverse change in dimension for correlating the location of afeature 'on the film structure after the change in its transverse dimension with respect to the location of said feature prior to the change indimension of the film structure. According to the present inventionthere are further providedprocess means for correlating the transverselocation thickness; characteristics of film structures after stretchingwith the transverse location of said features on the film structurebefore stretching.

According to the present invention there is further provided a methodfor controlling the thickness of a film structure of organicthermoplastic polymeric material that is stretched in at least one ofits two planar directions of length and width which comprises measuringthe thickness of said film structure at any location across the widththereof before stretching, measuring the thickness of said filmstructure at any location across the width thereof after stretching;computing the cumulative mass values of said film structure across thewidth thereof before and after stretching; adjusting the thickness ofsaid film structure at any point across the width thereof beforestretching responsive to deviation from a preselected value in thicknessof said film structure after stretching corresponding to the cumulativemass value of said film structure before and after stretching.

The nature and advantages of the invention will be more clearlyunderstood by the following description and the accompanying drawings inwhich:

FIG. 1 diagrammatically illustrates the apparatus of the inventiondepicting the relationship of its basic elements;

FIG. 2 is a schematic illustration of a change in transverse dimensioneltected by in-plane stretching of a film in its transverse direction;

FIG. 3 is a graphic illustration of the cumulative mass of a cast filmof polyethylene terephthalate plotted against a selected number of lanesacross the width of the film;

FIG. 4 shows graphically the percent deviation of the film thicknessdistributed across the width of the film of FIG. 3;

FIG. 5 is a graphic illustration of the lateral displacement inthickness characteristics of a film structure as effected by transversestretching;

FIG. 6 shows graphically the percent deviation of the film thicknessdistributed across the Width of the film of FIG. 5.

The apparatus herein disclosed in illustration of the invention includesan extruding die 11 adapted to continuously cast a substantially flatfilm structure 10 of organic thermoplastic polymeric material such as,for example, polyethylene terephthalate. The film structure 10 ispreferably continuously cast from extruding die 11 onto quench drum 12which may be cooled 'by circulating any suitable cooling mediuminternally thereof by any expedients well known in the art. Theextruding die 11 is provided with a plurality of adjustment devices 13a,13b, 13c 13n spaced across its width and so constructed and arrangedwhereby the opening of the slit-like die orifice (not shown) may beadjusted to control the thicnkess of the extruded film 10. From quenchdrum 12 the film 10 enters film heating device 14 which has a pluralityof separately adjustable, elongated, tubular radiant heaters (not shown)positioned close to the film structure so that the long axis of eachtubular radiant heater is parallel to and extends in the direction ofmovement of the film. Each heating element in heating device 14 isplaced in close, spaced-apart relationship across the width of film 10.A suitable heating device is that described in US. Pat. No. 3,161,711.The film structure 10 next advances to stretching apparatus 15 whereinthe film is heated to a temperature at which orientation occurs (anytemperature within the range specified in US. Pat. No. 2,823,421 issatisfactory) and the film structure is stretched in either of itslongitudinal or transverse directions, or in both directions, in anyknown manner as described in, for example, U.S. Pat. Nos. 2,823,421 and2,995,779. The film structure is preferably next heat-set in any manneras well known in the art as by subjecting the film structure toheat-setting tempertaures of 150 C. to 250 C., and thereafter the filmis advanced to a windup station whereat it is wound into roll form suchas 16.

4 The foregoing describes the film forming apparatus and operation inits most simple essential form, and it is the purpose and object of thepresent invention to provide improvements therein which will beoperative to provide film structures having improved thicknessuniformity or gauge across the width thereof.

The film thickness measuring gauge 17 is disposed between quench drum 12and film heating device 14. The thickness measuring gauge 17 is soconstructed and arranged as to traverse the width of continuouslyadvancing film structure 10 responsive to motive power derived fromelectric motor 18, and gauge 17 is adapted to generate an electricaloutput signal that is a function of the thickness of film structure 10.The film thickness measuring gauge is of well known constructionconsisting generally of a radiation source disposed on one side of thefilm structure and adapted to emit radiation such as beta rays, and areceiver or detector disposed on the other side of the film structurewhich is adapted to receive the radiation emitted by the radiationsource and provide an electrical signal which is proportional to thethickness of the film structure at the point of measurement thereof. Thefilm thickness measuring gauge ordinarily comprises a U- shaped bracketas shown in FIG. 1 supporting a radiation source 19 on one arm thereofand a receiver 20 on the other arm thereof. As shown in FIG. 1, theapparatus of the present invention is further characterized by filmthickness measuring gauge 21 having a radiation source 19' and being ofsimilar construction to gauge 17 but which is disposed between the filmstretching device 15 and the film roll 16. Film thickness measuringgauge 21 functions in the same manner as gauge 17 and is adapted totraverse the width of continuous advancing stretched film structure 10responsive to motive power derived from electric motor 22. The filmthickness measuring gauge 21 is adapted to generate an electrical outputsignal that is a function of the thickness of stretched film structure10.

For purposes of the present invention, as shown in FIG. 2, the filmstructure 10 is divided into a plurality of hypothetical lanes, 23a,2311, 23c 231/1 in the film before any change in film width andhypothetical lanes 24a, 24b, 24c 2411 in the film after stretching. Thesignal corresponding to the thickness of each preselected hypotheticallane of the film before stretching derived from gauge 17 is transmittedby conductor 25 to general purpose digital computer 26 which convertsthe analog film thickness functional signal to a digital film thicknessfunctional signal and stores the digital thickness signal of eachpreselected hypothetical film lane in a separate storage cell there-Within. Computer 26 is adapted to numerically integrate the storeddigital film thickness signals. The signal corresponding to thethickness of each preselected hypothetical lane of film structure 10after stretching derived from gauge 21 is transmitted by conductor 27 tocomputer 26 which also converts each signal to a digital film thicknessfunctional signal, and each such signal is stored in computer 26. Asuitable computer for this purpose is a Model 609 computer manufacturedby Information Systems, Inc. Skokie, Ill.

The apparatus of the present invention is further characterized byrotary switch 28 an rotary switch 29 each so constructed and arranged asto make one revolution for each preselected hypothetical lane traversedby gauges 17 and 21, respectively. Gauge 17 is operatively connected torotary switch 28 and gauge 21 is operatively connected to rotary switch29, and each gauge is adjusted to close each respective rotary switch atthe instant the scanning head of each gauge passes over the center ofthe preselected hypothetical lane. Each closed rotary switch momentarilycompletes the electrical circuit between respective power sources 30 and31 which transmit an electrical pulse to computer 26 for directing (bymeans of appropriate pulse-sensing circuits and relays within computer26) the digital film thickness functional signal to a storage 5 cell inthe computer corresponding to the hypothetical film lane.

The above-described film thickness gauges 17 and 21 are programmed totraverse film structure 10 a preselected number of times and the digitalfilm thickness functional signal of each preselected hypothetical filmlane obtained during each traverse is accumulated by the computer whichthen determines the average deviation of the film thickness based upon apredetermined reference thickness value (converted and stored in thecomputer in digital form) for each hypothetical film lane. The computer26 is programmed to compute the cumulative thickness values of a givennumber of hypothetical'film lanes, beginning from the :edge lane, andthe value thus determined is proportionalto the mass of the filmstructure from its edge to-the selected hypothetical film lane. Thecomputer 26 is programmed to compare the cumulative mass of the filmstructure, asabove indicated, before and after stretching, thus enablingthe computer to ascertain the location on the 'film structure beforestretching corresponding to the cumulative mass of the film structureafter stretching. It is thus possible by the above described sequentialcomparison of cumulative mass of the hypothetical film lanes to directadjustment signals to the appropriate one of adjustment devices 13 forchanging the orifice opening at that location on the extruding die 11thereby'to remedy any deviation in film thickness occurringat'thatlocation on film structure 10.

The foregoing is readily accomplished in the following manner, referringto FIG. 1. Computer 26 ascertains a film thickness deviation which mayvary from a preselected value or may vary from a sheet average basedupon the filmthickness functional signal itreceives, converts and storesfrom gauge 21. Computer 26 then determines the percentage of cumulativefilm thickness values (ie mass) tothe particular hypothetical film lanein the stretched film, and by successive comparison determines thelocation (lane) on the cast. film before stretching having the mostnearly corresponding cumulative mass value. The computer 26 nexttransmits a deviation signal which is a correction signal that is afunction of the film thickness deviation after stretching alongmultiple-conductor cable 32'to switch 33 where it may be re-directedeither along cable 34 for adjusting the opening of the orifice lips ofextruding die 11 by means of adjusters 13a, 13b, 13c 1311, or it may bedirected along cable 35 to heating device 14 for controlling thelongitudinal lane heaters. Adjustment of the orifice lips in extrudingdie 11 is effected by controlling the operation of a motor drive soconstructed and arranged as to open or close the orifice lips; suchmechanism is well known in the art and described in, for example, US.Pats. Nos. 3,122,782; 3,122,783 and 3,212,127. Satisfactory filmthickness control can'usually be achieved by adjusting the power inputto the-longitudinal lane heaters of heating device 14. For this purpose,heating device 14 is equipped with controller 36-which has motor-drivenvariable transformers calibrated to provide an appropriate change inpower to the heater in response to the magnitude of the deviation orcorrection signal which represents a specific deviation in the thicknessof the stretched film 10.

The advantages of the present invention are demonstrated in thefollowing illustration considered in connection with FIG. 3 and Table 1herebelow:

ILLUSTRATION No. 1

A polyethylene terephthalate film was produced by a continuouspolymerization process integrated with a casting and stretchingoperation to produce a biaxially oriented film. The molten polymer wasextruded from a hopper having a slit orifice and cast onto an internallycooled quench drum from which it was stripped and thereaftersequentially stretched, first in the machine direction (MD, direction ofextrusion) and then in the transverse direction (TD). The extrusionhopper was equipped with a plurality of adjustment bolts to adjust theclearance be tween the essential edges of the lips forming the slitorifice, thus controlling the thickness of the web. This adjustment wasused for gross manual control, but also was suitable for closed loopautomatic control. The stretching apparatus was equipped with a seriesof separately adjustable lane heaters positioned adjacent the web justbefore the first direction stretch, and disposed lengthwise in MD toprovide a parallel array across the web whereby thicker lanes could beselectively heated to alter the drawing characteristics and control thethickness uniformity of the sheet. Thickness measuring elements, asherein described, were employed to provide an output signal functionalof the thickness of the cast film in separate lanes across the web; andsimilar elements were employed to provide similar data on the stretchedfilm. These signals were fed to a computer which computed the deviationfrom a preselected thickness value, the cumulative percentage of mass ateach of the respective lanes from one edge across the film as indicatedin the table below, with means for comparison to determine the locationof equivalent masses and direct the deviation control signal to theappropriate lane for adjustment of the thickness controlling means.These data were printed out and could be used for manual adjustmentafter selection of the proper lane for adjustment from the printoutdata. In this operation the data were employed in a closed loopautomatic control system by the programmed computer which directedadjustment signals to the motor driven controls on each of the pluralityof separate individually adjustable lane heaters (see US. Pat.3,161,711) to adjust the heat input to a lane on the film beforestretching and establish a temperature differential between the thickerand thinner lanes.

Greater gauge uniformity was observed on separate samples of the sheetsby direct measurement after using this gauge control apparatus, thanwithout an automatic control apparatus, or with apparatus which directedthe adjustment signal from the detected gauge deviation to theadjustment means at a location on the web based on the fraction of thedistance across the web, rather than a location based on the fraction ofthe mass of the web. Improvement was more effectively shown, however, byan approximately 20% reduction over an extended period of time in thenumber of film rolls rejected for gauge related defects, such asdistorted rolls and gauge ban s.

'IABLE.-CUMULATIVE MASS AND PERCENT THICKNESS DEVIAIION OF POLYEIHYLENETEREPHTHALATE FILM Before stretching Percent Cumulative PercentCumulative Lane thickness percent Lane thickness percent No. deviationmass No. deviation mass TABLE Gontinued Percent Cumulative PercentCumulative Lane thickness percent Lane thickness percent N0. deviationmass N0. deviation mass After stretching 4. 6 1. 1 8. 7 51. 3 -5. 7 2. 13. 52. 4 6. 2 3. 2 2. 2 53. 6.6 4.2 -0.9 54.6 7. 3 5. 3 0. 1 55. 7 8. 66. 3 +1. 7 56. 9 9. 0 7. 3 +2. 4 58. 0 8. l. 8. 3 +3. 4 59. 2 7. 7 9. 4+5. 0 64. 0 6. 9 10. 4 +6. 3 61. 5 4. 5 11. 5 +7. 4 62. 8 1. 2 12. 6 +8.1 64. 0 +2. 6 13. 8 +8. 8 65. 2 +6. 5 15. 0 +9. 3 66. 4 +7. 8 16. 2 +10.7 67. 7 +9. 1 17. 4 +8. 3 68. 9 +10. 6 18. 6 +7. 3 70. 1 +1. 9 19. 8 +3.3 71. 2 -0.2 20.9 +1.1 72.4 3. 6 22. 0 +0. 2 73. 5 0. 1 23. 1 3. 7 74. 6+1. 4 24. 3 5. 0 75. 7 +1. 8 25. 4 4. 0 76. 6 +7. 2 26. 6 5. 3 77. 8+9.0 27. 8 6. 0 78. 9 +1.9 29.0 7.2 79.9 +0. 2 30. 1 8. 6 80. 9 3. 7 31.2 5. 7 82. 0 9. 0 32. 2 3. 4 83. 1 7. 8 33.2 3. 4 84. 1 4. 4 34. 3 1. 285. 3 3. O 35. 4 5. 7 86. 3 0. 5 36. 5 -7. 8 87. 3 +4. 3 37. 7 5. 7 88.4 +7. 1 38. 9 3. 8 89. 5 +3. 4 40. 1 +0. 7 90. 6 +10. 0 41. 3 +4. 2 91.8 +7. 3 42. 5 +7. 8 93. 0 1. 2 43. 7 +9. 4 94. 2 3. 3 44. 7 +10. 6 95. 56. 9 45. 8 +7. 8 96. 7 4. O 46. 9 +4. 9 97. 9 1. 4 48. 0 0. 6 99. 0 +2.0 49. 1 6. 3 100. O

What is claimed is:

1. A method for controlling the thickness of a film structure of organicthermoplastic polymeric material that is stretched in at least one ofits two planar directions of length and width which comprises:

measuring the film thickness in each of a plurality of hypotheticalplanes across the widths of the film before and after stretching;determining any average deviation of the film thickness in each laneafter stretching from a preselected value;

determining the cumulative mass value to a deviating lane by integratingthe measured thickness values from one edge of the film to the lane;

determining the location of the lane in the unstretched film which has acumulative mass value corresponding to that of the deviating lane in thestretched film; and

making the appropirate adjustment in a thickness controlling means whichaffects the determining lane in the unstretched film in order to bringthe deviating lane to the desired value.

2. The method of claim 1 including the further step of:

transmitting a deviation signal that is a function of the film thicknessdeviation for making the appropriate adjustment in the thicknesscontrolling means.

3. The method of claim 1 wherein the thickness of said film structure atany point across the width thereof before stretching is adjusted byvarying the opening of orifice lips in an extruding die from which saidfilm structure issues.

4. The method of claim 1 wherein the thickness of said film structure atany point across the width thereof before stretching is adjusted byheating said film structure and thereafter stretching it.

5. A method of manufacture for controlling the thickness of a filmstructure or organic thermoplastic polymeric material that is stretchedin at least one of its planar directions of length and width whichcomprises sensing the thickness of said film structure at any locationacross the width thereof before stretching;

transmitting first control signals that are a function of the thicknessof said film structure at any point across the width thereof beforestretching; sensing the thickness of said film structure at any locationacross the width thereof after stretching;

transmitting second control signals that are a function of the thicknessof said film structure at any point across the width thereof afterstretching;

accumulating said first control signals and said second control signalsfor computing cumulative mass values of said film structure across thewidth thereof before and after stretching;

determining any average deviation of the film structure thickness ateach location after stretching from a preselected value;

determining the cumulative mass value to a deviating location byintegrating the measured thickness values from one edge of the filmstructure to the location; transmitting a deviation signal to athickness control means for adjusting the thickness at any point acrossthe width of said film structure before stretching.

6. The method of claim 5 wherein the thickness of said film structure atany point across the width thereof before stretching is adjusted byvarying the opening of orifice lips in an extruding die from which saidfilm structure issues.

7. The method of claim 5 wherein the thickness of said film structure atany point across the width thereof before stretching is adjusted byheating said film structure and thereafter stretching it.

8. A method for manufacturing a film structure of organic thermoplasticpolymeric material of improved thickness uniformity across the widththereof which comprises:

measuring the thickness of the film structure at a plurality oflocations across the width thereof between a quench drum and a heatingdevice and before it is stretched and, thereafter, measuring thethickness thereof between a stretching device and a fihn roll and afterit is stretched;

determining any average deviation of the film structurethickness at eachlocation after stretching from a preselected value; determining thecumulative mass value to a deviating location by integrating themeasured thickness values from one edge of the film structure to thelocation;

determining the location in the unstretched film structure which has acumulative mass value corresponding to that of the deviating location inthe stretched film structure; and

making the appropriate adjustment in a film structure thicknesscontrolling means which aifects the determined location in theunstretched film in order to bring the deviating location to the desiredvalue.

References Cited UNITED STATES PATENTS 3,212,127 10/1965 Flook, et al.l8-2 I 3,347,960 10/1967 Fenley 26440 ROBERT F. WHITE, Primary Examiner.l. H. SILBAUGH, Assistant Examiner US. Cl. X.R.

18-2 HA; 250-833 D; 264-2l0 R, 289

